Tomography, both transaxial and longitudinal, has been used in diagnostic radiology for some time. The term transmission computed (axial) tomograph (CT refers to a diagnostic radiologic procedure which provides images of transverse sections of the body. CT permits accurate and non-invasive quantitative determination of the x-ray absorption properties of anatomical structure buried deeply in the human body or of the distribution of a radionuclide or of photon emission events in the body.
Generally in diagnostic positron emission CT, a patient is administered a radionuclide, causing millions of positrons to be emitted within the patient. These positrons travel for very short distances, no more than a few millimeters, and in their travel interact with electrons of similar mass. When the positrons and electrons interact, there is an annihilation which takes place and the annihilation event which occurs consists of the mass of the positron and electron being annihilated or disintegrated, with photons being emitted substantially at 180 degrees with respect to each other. The photons then travel the distance required to impinge upon detectors. In the detectors, the energy carried by the annihilation photons is converted to a flash of light, and it is this flash of light which is sensed by photomultiplier tubes located at the ends of the detectors which is a recording of the annihilation which took place within the subject.
In the space of a few minutes, hundreds of thousands of such "flashes of light" will be generated and electrical signals representing these flashes are processed by a system which may include a specially programmed digital computer so as to form an image of the area under examination. In the emission CT process of this invention only those annihilation photons travelling within certain planes (or slices), are counted in such reconstruction.
In computerized tomographic reconstruction, measurements are taken of the radiation intensity along each of a great number of straight paths which are all in the same plane, and this same plane is what defines the planar, tomographic section or slice. If this slice is considered to be divided into a two-dimensional matrix of slice elements by an imaginary grid, then each of those elements is intersected by a number of the paths. The measurements include one for each of these paths, and they can be processed in accordance with the techniques disclosed, for example, in U.S. Pat. Nos. 3,778,614 and 3,924,129 to form a map of a radiation-related characteristic of these slice elements. If X-ray radiation propagates along these paths, the characteristic is typically the X-ray absorption of each slice element. If the radiation is gamma photons or annihilation photons, the characteristic is the number of such photons emitted due to events occurring in the slice element (per unit time).
When this invention is practiced, the detectors of photons and the determinations of the paths of the detected photons are used to determine the number of such photons (per unit time) detected along each of these paths. The paths can have an arbitrarily selected width (their thickness or the thickness of the slice being imaged). A typical width in my invention may be of the order of mm, or at most up to a few cm. A typical thickness may be of the same order.
For determining the radiation intensity, a beam of radiation is produced and is collimated to a pencil beam or to a planar fan beam. This beam passes through the patient and, after further collimation, impinges upon a scintillation detector consisting of at least one luminescent crystal optically coupled to a photomultiplier tube. This tube provides electrical signals corresponding to the amount of radiation impinging on the crystal. The X-ray tube and the detector are connected rigidly. They scan the object to be examined in a linear translational motion called a transverse. After one such transverse, the gantry supporting the source and the detector steps rotationally (e.g., 1 degree) about an axis perpendicular to the section to be imaged, and another transverse occurs. Typically, this operation is repeated to cover a total of at least 180 degrees. The measurements of radiation made by this technique can be considered as a series of profiles of the attenuation of X-rays in the matter traversed at different angles. It is from these profiles that the image of the anatomy of the tomographic section is reconstructed. Any difference in X-ray attenuation to be reconstructed in the image must appear in the profiles.
The radiation transmission profiles which are acquired by the CT detector system are recorded in a digital form. From this, the CT system generates the image of the section. A number of different techniques have been used in tomographic reconstruction, such as those disclosed in said U.S. Pat., Nos. 3,778,614 and 3,924,129.
The reconstruction technique to be used in my invention is known as the "filtered back projection" reconstruction technique detailed in U.S. Pat. Nos. 3,924,129 and 3,778,614 and in the above references 4 and 5. The radiation detectors often used for this purpose are scintillation crystals, e.g., activated sodium iodide crystals coupled to photomultipliers or photodiodes, although xenon detectors and other crystals have also been used.
A number of radionuclides decay through the emission of positrons. One physical characteristic of these particles is highly serendipitous for the CT imaging of the radionuclides which emit them.
Positrons are positively charged electrons usually emitted by radionuclides which are unstable because they include an excess of neutrons with respect to a stable state. Positrons lose their kinetic energy in matter in a manner similar to that of electrons. However, as briefly described above, when positrons are brought to rest they undergo the phenomenon of annihilation, whereby the positron interacts with an electron, the two particles undergo annihilation, and the masses are converted into energy in the form of two photons called the annihilation photons. These two photons travel at about 180 degrees from each other and each carries an energy of approximately 511 keV. It is through the simultaneous detection of the two annihilation photons that positron-emitting radionuclides are of significance in CT reconstruction.
The annihilation radiation can be uniquely detected by two scintillation detectors connected to a coincidence circuit. In this scheme, a count is recorded only if both detectors detect the annihilation photons simultaneously. This method of detection provides an "electronic" collimator since the annihilation events occurring outside a straight line joining the two detectors cannot be recorded (except in a statistically insignificant occurrence) because the annihilation photons are emitted at about 180 degrees from each other. Thus, two detectors operated in coincidence establish a field of view encompassed by the lines joining them.
The advantages of positron emission CT have been recognized, (see references 17 and 19-24), and several systems have been designed and tested. The overwhelming majority of these designs incorporate scintillation or imaging detectors. Hereinafter, the term detector shall be used to describe any detector useful in nuclear medicine imaging techniques. In its simplest form, a positron emission CT system consists of two detectors facing each other and scanning across the object at different angles. In order to achieve high efficiency in collecting the radiation, more detectors can be placed around the object. Another design for this purpose consists of a circle of detectors rotating around the object to be imaged.
It should be noted that the diagnostic tomographic visualization of an organ typically requires several tomographic sections. Thus, tomographs capable of yielding only one section at a time must be operated sequentially with relative motion of the tomograph with respect to the patient between sections. This approach is wasteful of radiation, time consuming, and often unsuitable for the study of time-dependent dynamic phemonena throughout the organs images. Furthermore, the accurate indexing of the apparatus with respect to the patient is difficult. To alleviate this difficulty state-of-the-art positron imaging systems incorporate the ability to provide several sections simultaneously. Thus, the MGH positrion camera does provide a number of simultaneous CT sections (see reference 26).